In digital multimedia systems, individual data streams, for example video and audio data as well as data channels, etc., are digitized at different sampling rates. In this case, the digital data streams have no obviously common timing base. To resynchronize individual data streams in a receiving unit after they have been transmitted, both a common timing base for the individual sampling rates and synchronization points for aligning the individual data streams with each other are needed.
The Moving Picture Expert Group (MPEG) standard defines not only coding techniques for video and audio data streams but also syntactic and semantic rules for packetizing coded data streams, making it possible to subsequently identify the individual data packets of each packetized elementary data stream (PES) and present them in the correct time sequence with respect to one another.
The MPEG standard is explained, for example, in D. Teichner; der MPEG-2-Standard (The MPEG 2 Standard) in: Femseh-und Kinotechnik, Volume 48, Number 4/1994, pages 155 to 163. The MPEG systems require a single time base reference for coding and decoding. For this purpose, a counter based on a 90 kHz frequency, known as a program clock reference (PCR) counter, is provided in the transmitter and receiver. A presentation time stamp (PTS), derived from the 90 kHz counter in the transmitter, is added to each elementary data packet to describe the time at which the data packet is presented in relation to the instantaneous status of a program clock reference (PCR) counter in the receiver.
The system time clock (ftransmitter) of the transmitter is transmitted by continuously streaming the data of a system clock reference counter to the receiver. The latter is operated directly by the system frequency. The program clock reference (PCR) counter is needed to synchronize the individual data streams (audio, video) with each other. To do this, the system time clock (ftransmitter) is divided down to the count frequency of the program clock reference counter and supplied to the program clock reference counter. The status of the PCR counter is then inserted into the individual PES data packets as a numerical value in PTS form. For transmission purposes, the different PES values for audio, video, data channels, etc. as well as the values of the system clock reference counter are then combined into a common data stream, known as the transport data stream.
The problem with synchronizing the various individual data streams reconstructed from the transport stream in the receiving device is that the values of the system clock reference counter must be transmitted from the transmitting device to the receiving device with constant delay throughout the entire system. Based on the values of the system clock reference counter and their time intervals, it is possible to recover the system clock (ftransmitter) of the transmitter in the receiving device. This system time clock (STC) of the receiving device clocks an STC counter at the receiving end. The frequency of the system time clock (STC) is supplied on the basis of the difference between an incoming value of the system clock reference counter and the status of the STC counter. At the same time, this system time clock (STC) is used to generate the receiving-end sampling frequencies (f′audio, f′video, f′data) for the different data streams (audio, video, data channels). The latter thus have the same frequency as the corresponding sampling frequencies (faudio, fvideo, fdata) in the transmitting device.
To synchronize the individual data streams, the latter are aligned with the common timing base of the system time clock (STC) counter on the basis of their PTS flags. The output time of a specific item of data in a data stream is then determined separately from the output buffer, using the system time clock counter and the PTS flags for each of the individual data streams.
The problem with synchronizing the system frequency between the transmitter and receiver is that the values of the system clock reference counter must be transmitted with a constant delay from the transmitting device to the receiving device to make sure that the system time clock (STC) is recovered correctly in the receiving device.